1. Technical Field
This invention relates to the field of ceramic-based foams. More particularly, it relates to a method of obtaining ceramic-based foams that exhibit improved gel strength in the pre-fired green state.
2. Background of the Art
Porous refractory materials are in demand for a wide variety of industrial and consumer applications. These include, for example, catalyst supports, flame supports, gas filters, air fresheners, ceramic armor, diesel particulate traps, insulation materials, artificial parts for the body such as heart stents, metal filters, reusable filters, liquid filters, storage and transportation for flammable and/or toxic materials, humidity sensors, chromatography, filter candles for filtration of hot combustion gases, diaphragms, membranes, refractory separators, phase dividers, and electrolytes for high temperature fuel cells. Methods to make such porous refractory materials generally include formation of a foam of some type, wherein gas is introduced into or generated within a slurry or dispersion, allowed or induced to expand therein, and the slurry or dispersion material then forms an interconnected porous structure. This structure is then allowed to dry, harden and/or cure, such that the porous refractory material becomes useful.
For example, in WO95/30641 (PCT/GB95/01063), a method of making a porous refractory article is described. That method comprises the steps of forming an aqueous dispersion of refractory particles including a polymerizable monomer component which on polymerization generates an exotherm; adding a thermally-activated gas generating substance; and adding agents such as initiator and/or catalyst to facilitate the polymerization. The gas forms bubbles under the heat of the exotherm, and an interconnected open pore structure results.
In another method, described in GB 2 317 887 A, a porous article composed of bonded particles, such as hydroxyapatite or the like, is formed, beginning with a dispersion comprising a liquid carrier, a polymerizable monomer, and the particles. Thus, the polymeric portions of the porous structure include the particles bonded therein.
A problem that is encountered in many methods that involve formation of an initial foam, however, is that the foam is relatively fragile, and may tend to collapse easily during the period between its initial formation and its final form. This is encountered particularly in the case where the initial slurry or dispersion is water-based. This is because, by definition, most methods require at least some drying of the foam, and this loss of water changes the dynamics of the foam, leading to its collapse. Because of this problem, water-based foams often require special types of handling and further processing, which may add to the expense and difficulty of producing them.
In view of the special difficulties encountered in preparing water-based porous refractory articles, it is desired in the art to find new and improved methods of preparing these foams wherein the capability of the foam to maintain its shape throughout the formation of the final porous article is improved.